This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Engineering proteins for enhanced stability is often critical for their industrial or pharmaceutical use, but experimental and computational efforts to accomplish this goal have met with limited success. We identified stabilizing mutations in a WW domain based solely on a measurement of protein fitness for more than 47,000 variants, and the use of these fitness measurements to calculate more than 5,000 epistastic relationships. We introduce a new metric, termed ?partner potentiation,? which is a measure of the mean epistasis effect that a mutation imparts to other mutations when combined in a double mutant. This metric allowed us to infer the identity of 15 stabilizing mutations within the WW domain.